Solid state lighting (SSL) elements such as LEDs are rapidly gaining popularity as the successor of more energy-inefficient lighting devices such as incandescent lighting devices. However, market penetration of SSL elements is hampered by a number of factors.
First of all, the intrinsically different nature of light generation by such elements means that the appearance of an SSL element-based lighting element can be quite different to the appearance of an equivalent incandescent lighting device. This extends to the luminous distribution generated by such devices; whereas an incandescent lighting device produces a luminous distribution that is largely omnidirectional in nature, SSL element-based lighting elements naturally tend to produce a more directional luminous output.
Secondly, the manufacturing cost of SSL element-based lighting devices is significantly higher than the manufacturing cost of incandescent lighting devices. Although this higher cost is offset by the superior lifetime of the SSL element-based lighting devices, it nevertheless dissuades consumers from purchasing SSL element-based lighting devices as the initial cost may be considered prohibitive.
The above problems pose a fundamental dilemma for the manufacturers of SSL element-based lighting devices, because in order to more closely match the appearance of incandescent counterparts additional optical elements have to be added to the design of the lighting device. However, such additional optical elements can significantly add to the cost of the SSL element-based lighting device. This problem is particularly relevant when the lighting device is a light bulb, especially light bulbs that are expected to have a traditional appearance such as candle-shaped light bulbs
An example of such an optical element is disclosed in WO2010/079439 A1. This application discloses an optical element including a light guide into which light from one or more light-emitting diodes in a light unit arranged at one end of the light guide is injected, and a reflector arranged at the other end of the light guide capable of reflecting light incident on the reflector. The one or more light-emitting diodes are arranged in the centre at one end of the light guide.
Although this optical element ensures that the luminous distribution closely mimicking that of an incandescent light bulb is achieved, e.g. it can closely mimic the sparkling effect of an incandescent candle light bulb, a drawback is that the design of the optical element is closely linked to the number of light-emitting diodes in the lighting device where the optical element is to be used. The limited reusability of such optical elements is a contributing factor in the relatively high cost of SSL element-based lighting devices.